The present invention relates to a method and an apparatus for forming a disk-wheel like formed parts using a vertical die casting machine or a vertical squeeze casting machine.
For example, casting works for aluminum disk wheels for automotive vehicles are very often carried out using the vertical die casting machine because of less involvement of gas at the time when hot molten metal is injected or for any other reason. FIG. 1 is a longitudinal cross sectional view schematically showing a metal mold and an injection unit provided in a die casting machine of this kind which has been used in the art. Such a conventional die casting machine will be described in conjunction with this figure. Onto a fixed platen 1 fixed on the machine base, a fixed metal mold 2 having a cylindrical projection in its central portion is affixed. Onto a movable platen 3 supported by a mold clamp cylinder (not shown) to move upwardly and downwardly, a movable metal mold 4 having a low projection in its central portion is affixed. A plurality of cores 5 are inserted into the space between the both metal molds 2 and 4 from their circumferentially equally dividing positions so that there are movable in a horizontal direction. These cores 5 are fixed to a piston rod 7 of a cylinder 6 supported on the side of the movable platen 3 and advance or withdraw in a horizontal direction by hydraulically advancing or withdrawing the piston rod 7. By the both metal molds 2 and 4 and the closed cores 5, a cavity 8 is defined. An injection sleeve 9 is fitted into a sleeve hole formed through the fixed platen 1 and the fixed metal mold 2 from the lower direction so that it can be inserted thereinto or detached therefrom. A plunger chip 10 which advances or withdraws by an injection cylinder is fitted into the injection sleeve 9 so that it can advance or withdraw. A hot molten metal 11 is poured into the injection sleeve 9 under condition where the injection sleeve 9 is detached from the sleeve hole.
In the above-mentioned arrangement, when the injection sleeve 9 is fitted into the sleeve hole with the hot molten metal 11 being poured into the injection sleeve 9, thereafter to advance the plunger chip 10, the hot molten metal 11 is injected into the cavity 8. Thus, after the hot molten metal is solidified and cooled, the movable platen 3 is caused to move upwardly thus to conduct the opening of the mold and the cores 5 are opened in an outward direction, thus to extrude a product having been solidified within the cavity 8 using a product extruding device (not shown) to take out the product toward the outside of the die casting machine.
In such a die casting work, unless casting of the hot molten metal is carried out for a relatively short time, temperature of the hot molten metal lowers to increase the viscosity. As a result, this causes poor circulation of the hot molten metal within the metal mold cavity, thus allowing the quality of the formed part to be lowered. For this reason, it is necessary to cast the hot molten metal as short as possible. Contrary to this, according as the injection speed is increased, the surface of the hot molten metal within the injection sleeve or the metal mold cavity becomes disturbed, whereby it is brought into the dispersed condition. Thus, the involvement of such an atmosphere leads to inclusion of blowholes in the forming process. This may cause occurrence of nests in the formed part or other unfavorable phenomena, resulting in high possibility that problems will occur in respect of strength, pressure resistance and liquid-proofing property.
In view of this, it is the present state that forming of disk wheels is carried out in dependence upon a so called squeeze casting method in order not to disturb the flow of hot molten metal within the metal mold as far as possible and to avoid the involvement of gas to the utmost in the process of forming them using the vertical die casting machine, the squeeze casting method being characterized in that the advancing speed of the plunger chip for injection is set to a relatively low speed of approximately 20 to 100 mm/sec and in that casting is carried out at a fixed speed from the beginning of the injection to the time when the hot molten metal is completely filled up.
With the method of forming disk wheels based on such a squeeze casting method, for example, when the diameter of the disk wheel is about 13 to 14 inches and the means thickness at the time of forming is about 5 to 6 mm, high quality and high strength disk-shaped products can be produced. However, when an attempt is made to allow the thickness to be less than 4 mm with a view to thinning the thickness for realization of lightness, the above-mentioned conventional forming method has the problem that even if heat insulation of the metal mold and the like may be strengthen as far as possible, circulation of the hot molten metal is extremely degraded, resulting in very difficulty in forming.